In the Qing Dynasty, letters were passed down by calligraphy, with rigorous writing and formal expression. Letters in the late Ming Dynasty are relatively free, with clear text structure, clear meaning and wonderful records that pay attention to details.
Footnote is a word, pronounced chǐ dú, which refers to a foot-long book list used by the ancients to write; Letter, letter; Words or ink, handwriting. According to records, there are several specifications of wooden slips, but they are about one foot long, so they are called "ruler slips" and are mostly used in calligraphy and painting.